Breath alcohol measuring devices are known, for example, from DE 43 27 312 C2 and EP 1 371 982 A1. The test subject blows breathing air via a mouthpiece into such breath alcohol measuring devices, and the breathing air is then sent through a measuring channel, in which a signal representative of the tidal volume flow is generated, usually by a pressure sensor, and the signal is sent to a control and evaluating unit. This tidal volume flow signal is needed, on the one hand, to make it possible to monitor the interruption-free breathing of the test subject. Furthermore, the tidal volume flow signal is integrated so that a preset minimum volume can be determined, which is needed for a reliable measurement result, because a sufficient percentage of the breathing air needs to originate from the depth of the lungs in order to make it possible to infer the blood alcohol concentration from the breath alcohol concentration.
A defined sample volume is drawn off from the measuring channel by means of a pumping means and fed to an electrochemical sensor. Ethyl alcohol is reacted at the electrodes of the electrochemical sensor, and an electric current is generated, which slowly subsides after a rapid rise, which corresponds to the subsiding reaction. The sensor current is integrated for a preset period of time in the control and evaluating unit, and the overall charge is estimated from this integrated charge value, from which the breath alcohol concentration can be derived.
Two methods are available, in principle, for the calibration of breath alcohol measuring devices. On the one hand, a test gas of a known composition is blown from a pressure cylinder into the breath alcohol measuring device or, on the other hand, an air flow, which is passed through an alcohol-water mixture of constant temperature (usually 34° C.), is blown into the breath alcohol measuring device. One speaks of using wet gas in the case of the latter method, because the air is enriched not only with alcohol, but also with water vapor. Both procedures have specific advantages and disadvantages in practice. Test gas in pressure cylinders can be transported more easily and requires no devices for heating and temperature control. The drawback of the use of test gas is the dependence of the result on the barometric pressure. A corresponding correction of the results and consequently of the calibration is necessary in case of deviations from the normal pressure. On the other hand, even though wet gas is substantially independent from the outside pressure and more similar to the breathing air, it does require a heating means and a heated gas carrying system to avoid condensation, so that dry gas is used increasingly frequently in practice.
A problem arises in connection with the use of the two procedures described due to the difference in the sensitivity of the electrochemical sensor to dry gas, on the one hand, and wet gas, on the other hand: At equal alcohol concentration, dry gas leads to a measured value that is lower by about 10% than wet gas. Half of the effect can be explained by the absorption of the water in the sample on the sensor surface, but the cause of the other half of the effect is still unknown.
The breath alcohol measuring device must therefore be informed in practice by an input of the type of gas used for the calibration or the routine testing. The device must have special input means for this purpose. Considerable errors in measurement and consequently considerable calibration errors may result from mistakes or from the incorrect entry of the test gas.
The test subject, i.e., the user, must blow into a breath alcohol measuring device in case of the use of breath alcohol measuring devices in so-called interlock systems, after which a means arranged downstream, e.g., access to a machine or the starter of a motor-driven vehicle, is either released or blocked depending on the measured breath alcohol level. The recognition of attempts at manipulation, e.g., blowing through an activated carbon filter, is of great significance in such interlock systems. Such attempts at manipulation have hitherto been recognized by additional technical measures, e.g., the measurement of the breath temperature or the measurement of the moisture content in the breathing air blown in with a moisture sensor. However, this is associated with increased technical effort.